Skis have various characteristics that affect the ease of skiing and the ability of the skier to achieve or to fail to achieve a high level of proficiency. Thus, a good ski exhibits straight line stability at high speed, and also the ability to turn easily and to absorb bumps and ripples in the snow. In addition, it should show an ability to traverse across a steep slope without side slip.
Skis include a core, which may be made of any of various materials, such as wood, foam, honeycomb, and various laminated materials. The core controls most of the resilience of the ski.
The bottom surface of the ski is preferably made of a material which is slick, such as a suitable plastic that slides well over the snow, and this bottom portion is bonded to the bottom of the core.
The sides and the top of the ski may be of wood, or of plastic, or metal, but are usually of a type of material different from the bottom, because slipping and sliding is not their function. The sides and top are also bonded to the core, and are preferably surrounded with a waterproof covering, which may also provide decoration.
The bottom is usually provided with metal edges that function to cut into the ice or hard snow so that the ski can bite and can hold a turn without sliding sideways. These metal edges also help when traversing a slope, and when the skier wishes to stop. The skier himself causes these metal edges to bite into the snow by angling his legs, and thus the skis, in the direction of the turn.
Good skis are usually narrower in the center than at the tips when viewed in plan, so that the sides to which the metal edges are affixed form a large arc. This arc helps to cause the ski to start turning when it is angled. The longer the portion of the metal edge that cuts into the snow, the more lateral force the ski can exert to enable the skier to turn sharper and more quickly. Skis are limber and do not twist uniformly along their length when the skier angles them to turn.
Heretofore, such skis have been weak in torsion, so that the tips, the front, and the rear would not angle as much as did the center where the ski boot is attached, as by bindings. This weakness in torsion has forced skiers to accentuate the angularity of their legs, and the ends of the ski did not achieve the same edge hold that would have been obtained if the ski had not twisted. Such a twisted ski could not exert the amount of force on a turn that an untwisted ski could, and thus became a source of inefficiency.
Another important quality in skis is their compliance, their flexurability in the vertical direction. A highly compliant ski makes the ride smoother over the snow, enables the skier to maintain his balance more easily, and achieves a relatively even distribution of pressure along the length of the ski as applied to its bottom surface. Pressure along this bottom surface is a factor in making skis run fast. Areas of extreme pressure due to low compliance are certainly undesirable.
While a ski could be made to be very stiff in torsion by making it much thicker, it would then be much less compliant when moving over ripples and bumps and deep depressions in the snow, so that the overall result would be unsatisfactory. The compliance of a ski relates to its stiffness or flexibility in beam. A very thick ski would be relatively stiff and not compliant. However, it is desirable to have high compliance, and so it is desirable for a ski to be relatively weak in beam. On the other hand, twisting of the ski takes place because a typical ski is very weak in torsion. Both types of action --compliance and torsion -- may occur separately or simultaneously, depending on the terrain and on the action of the skier.
In the past skis have been relatively weak in both beam and torsion. It would be easy, as indicated earlier, to make such a ski stiff in both beam and in torsion, but it has been believed that it would be nearly impossible to make the ski weak in beam and yet stiff in torsion. However, this is what is basically desired in a ski.
An object of the present invention is to accomplish stiffness in torsion, while leaving the beam flexible, or relatively weak, so that the beam strength is relatively low, but the torsion stiffness is high.
Another object of the invention is to enable a designer to control each of the two factors, torsion and beam strength, almost independently of each other. Usually this can be effected by starting with a ski design that is weak in beam and weak in torsion, and then by applying the principles of the present invention, the torsional rigidity can be increased without substantially affecting the beam flexibility.